Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography. Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
Recently, various printers allowing higher-speed printing have been developed and commercialized increasingly in the field of the dye-diffusion transfer recording systems. The high speed print enables to shorten a waiting time when a user obtains a print in a shop. For the foregoing reason, there is a demand for further speeding up of printing.
In order to prevent thermal sticking between a thermal printer head of a printer and an ink sheet and to give the thermal printer head and the ink sheet a slipping property therebetween, a heat-resistant lubricating layer is formed on the ink sheet surface contacting the thermal printer head. The thermal sticking occurs in a case where there is a break of the ink ribbon or the slipping property of the ink ribbon is insufficient when an image is printed, whereby image defects may be generated because the ink sheet is stretched or creased, or deformed into some other form at the printing time. According to high-speed printing, a thermal printer head comes to contact the heat-resistant lubricating layer at a higher temperature and a higher speed. Thus, the heat-resistant lubricating layer is desired to have even better performances.
For example, Japanese Patent No. 2,591,636 discloses a technique of forming a hard heat-resistant lubricating layer having a high crosslinkage density mainly from polyisocyanate, thereby giving heat resistance and lubricity to the layer. Moreover, Japanese Patent No. 3,410,157 discloses a technique of incorporating a phosphate ester-series surfactant excellent in lubricity into a lubricating layer, and further incorporating, into the layer, magnesium hydroxide and particles having a Mohs' hardness less than 3 as neutralizing agents in order to restrain corrosion or abrasion of a thermal printer head by decomposition of the phosphate ester.
The inventors have investigated improvements on performances of a heat-resistant lubricating layer of an ink sheet by use of the techniques described in the above-mentioned Japanese Patents, in high-speed printing. As a result, the inventors have found out that a new problem that the transferred dye density falls is caused although the lubricity between the heat-resistant lubricating layer and a thermal printer head can be certainly kept. In order to compensate for the fall in the transferred dye density, it is thought that the amount of the colorant (dye) contained in the dye layer is increased, but it has a problem that costs increase.